A new member of the ets family of genes has recently been identified (ERP for ets-repressor-protein) by its interaction with an ETS2 promoter regulatory element. This gene has been shown to repress transcription in transient transfection assays from a number of promoters that contain an ets recognition sequence, including ETS2 and retroviral LTRs. To study the role of ERP in vivo, ERP was subcloned into a variety of eukaryotic expression vectors, namely, pSG5 (SV40 constitutive promoter), pMSG (dexamethasone inducible promoter) and LdK (tat-inducible HIV LTR). Two other ERP hybrids were also used, the first containing the ERP binding domain fused to the ETS1 activation domain (E1NS45), and the second containing the ETS1 binding domain linked to the ERP repressor domain (E1dPX45). In earlier studies, where the ERP repressor domain was transferred to a heterologous binding domain (GAL4), it was shown to be capable of repressing transcription in a DNA binding dependent way. Those ERP hybrids were also subcloned into the aforementioned vectors. HeLa cells were stably transfected with ERP and its hybrids to study the effect on the expression of other ets family genes, as well as genes that contain ets binding sites in their promoters (p53). In addition, stable clones of ERP and its hybrids were obtained in HeLa CD4+ in order to evaluate in vivo the effect of ERP on the HIV LTR following infection with the HIV virus.